Not applicable.
Not applicable.
The present invention relates to a plurality of chemical reactors arranged to moderate the intensity of a hot spot in at least one of the reactors.
Large quantities of natural gas are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, a significant amount of natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane, the main component of natural gas, as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into the higher hydrocarbons.
Current industrial use of methane or natural gas as a chemical feedstock proceeds by the initial conversion of the feedstock to carbon monoxide and hydrogen by either steam reforming (the most widespread process), dry reforming, autothermal reforming, or catalytic partial oxidation. Examples of these processes are disclosed in GUNARDSON, HAROLD, Industrial Gases in Petrochemical Processing 41-80 (1998), incorporated herein by reference. Steam reforming, dry reforming, and catalytic partial oxidation proceed according to the following reactions respectively:
CH4+H2O⇄CO+3H2xe2x80x83xe2x80x83(1) 
CH4+CO2⇄2CO+2H2xe2x80x83xe2x80x83(2) 
CH4+xc2xdO2xe2x86x92CO+2H2xe2x80x83xe2x80x83(3) 
Catalytic partial oxidation (CPOX) has recently attracted much attention due to significant inherent advantages, such as the fact that heat is released during the process, in contrast to the endothermic steam and dry reforming processes.
In catalytic partial oxidation, a hydrocarbon feedstock is mixed with an oxygen source, such as air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. When the feedstock comprises primarily methane (e.g., natural gas), the approximately 2:1 H2:CO molar ratio achieved by partial oxidation is generally more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch synthesis. An example of a Fischer-Tropsch process is disclosed in U.S. Pat. No. 6,333,294 to Chao et al., incorporated herein by reference.
Unfortunately, the heat production during a CPOX reaction can be a double-edged sword. In a CPOX reactor, in addition to the desirable selective partial oxidation reaction, there often occurs a substantial amount of undesirable reactions such as the non-selective oxidation of methane (e.g. to products other than CO and H2, for example, C, CO2 and H2O). Non-selective oxidation is much more exothermic than the desirable selective CPOX reaction (CH4+xc2xdO2xe2x86x92CO+2H2) and, therefore, produces much more heat. This heat produced by the desirable partial oxidation reactions and the undesirable non-selective reactions can sometimes have undesirable consequences.
DEUTSCHMANN ET AL., Natural Gas Conversion In Monolithic Catalysts: Interaction Of Chemical Reactions And Transport Phenomena, 6th National Gas Conversion Symposium, Girdwood, USA (2001), incorporated herein by reference, predicted that the hot spot in a monolithic syngas reactor, the catalyst surface temperature can be as high as 2000K. This hot spot can cause active phase transformation and/or sinter the catalyst, causing a loss of surface area and, consequently, a loss of catalytic activity. This loss of catalytic activity can, in turn, lead to an increase in the rate of non-selective reaction, causing faster heat liberation and an even quicker deactivation of the catalyst, thus perpetuating a spiral of deactivation. Thus, there is a desire to limit the presence of and moderate the intensity of hot spots in the reaction zone.
An example of a single pass reactor is shown schematically in FIG. 2. It includes reactor 70, feedstream 10, reaction zone 60, and product stream 30. Feedstream 10 enters reactor 70 and reacts in reaction zone 60 to form product stream 30. Preferably, reactor 70 is a syngas reactor. Accordingly, feedstream 10 preferably comprises an oxygen-containing gas such as air, oxygen-enriched air, or substantially pure oxygen, and a hydrocarbon-containing gas such as methane or natural gas. Feedstream 10 is preheated and passed through reaction zone 60. Reaction zone 60 preferably comprises a supported catalyst comprising rhodium. The feedstream 10 reacts to form product stream 30 which comprises synthesis gas. It is often observed that the temperature profile in, for example, a single pass catalytic partial oxidation reactor such as the reactor of FIG. 2, an example of which is shown in the graph of FIG. 6, has a temperature spike in the front half of the reaction zone followed by a drop in temperature in the second half of the reaction zone.
As is shown in FIG. 1 prior attempts to solve the problem of hot spots in the reaction zone have led to reactor designs, such as reactor 40, in which feed stream 10 enters reactor zone 1 where it reacts exothermically to produce intermediate stream 20, is redirected to reactor zone 2 where it reacts more mildly, and exits reactor zone 2 as product stream 30. The extreme temperatures of the front of the reactor zone 1 are moderated by the lower temperature of the same flow stream at the rear of the reactor zone 2. Other examples of reactors in which the flow is folded in on itself is shown in FIG. 3 of REDENIUS ET AL., Millisecond Catalytic Wall Reactors: I. Radiant Burner, AICHE Journal (May 2001) and FIG. 1 of IOANNIDES AND VERYKIOS, Development of a Novel Heat-Integrated Wall Reactor for the Partial Oxidation of Methane to Synthesis Gas, Catalysis Today (1998), both references incorporated herein by reference in their entirety. FIG. 3 of REDENIUS shows a reactor configured such that the reactor spirals around itself. FIG. 1 of IOANNIDES teaches a reactor in which the reactor doubles back against itself and an exothermic combustion catalyst is positioned adjacent an endothermic reforming catalyst, thus causing heat to transfer from the exothermic reaction zone to the endothermic reaction zone. With reference to FIG. 1, these types of configurations are disadvantageous because (1) all of the gas passing over reaction zone 1 must pass over reaction zone 2, thus making it impossible to vary the flow over both zones independently and (2) there is a gap between the two reaction zones 1 and 2 where the intermediate gas stream 20 may react non-selectively due to the absence of a catalyst to catalyze the preferred selective reactions. As noted above, the undesirable non-selective oxidation reactions produce more heat than the desired selective oxidation, thus this area of non-selective reaction will not only decrease the yield of the desired products, but also undesirably increase the amount of heat liberation.
Embodiments of the present invention run chemical reactions in adjacent reactors to ameliorate and reduce the effects of the hot spots generated by the reactions. Additionally, it is often the case that the present invention will advantageously decrease the rate of catalyst deactivation and increase catalyst life.
In a preferred embodiment of the present invention a chemical reactor which has a temperature spike in the first half and an area of milder temperature in the second half of the reactor, such as a partial oxidation syngas reactor, is arranged in an adjacent counterflow configuration with at least one substantially identical reactor. Thus, the temperature profiles of both reactors are moderated.
Other embodiments include running at least two partial oxidation reactors which are arranged in a manner such that the sharp spike in the temperature (i.e., the hot spot in the front of the reaction zone) of one reactor is greatly reduced and moderated by heat transfer to a cooler area in an adjacent reactor. Thus, many of the problems associated with the hot spot (e.g., catalyst deactivation) are greatly reduced, leading to longer catalyst life or higher average catalyst activity or both.
The temperature profile of two individual reactors and two reactors in accordance with an embodiment of the present invention is illustrated in FIG. 7. FIG. 7 shows individual temperature profiles (Ts1 and Ts2) of the reaction zones of two adjacent adiabatic counterflow reactors if there is no cross reactor heat exchange, and the combined temperature profile (Tsc) of the two counterflow reactors if heat transfer occurs between the two reactors. It should be noted that the height of the spikes in the temperature in the combined profile (Tsc) is much lower.
In another embodiment, at least two reactors are arranged in an adjacent counterflow configuration and the reaction zones of the two reactors are offset such that the hot spot of one reactor is moderated by a lower temperature in the other reactor.
In yet another embodiment, at least one reactor housing an exothermic reaction and at least one reactor housing an endothermic reaction are arranged adjacent to each other such that heat is exchanged between the two reactors and the heat of the exothermic reaction is moderated by the cooler endothermic reaction.
In another embodiment two cocurrent reactors are arranged such that the hot spot of one reactor is moderated by a cooler spot in the other reactor.